1. Field of the Invention
The invention relates generally to solar cells and pertains, more specifically, to a process for reducing the series cell resistance associated with thick film, electrical, metal contact systems of solar cells utilizing a soldering flux etchant.
2. Description of the Prior Art
In recent years the development of solar cell technology for terrestrial applications has accelerated rapidly. Solar cells may be generally defined as photovoltaic devices that directly convert sunlight into electrical current. The rapid development in terrestrial solar cell technology is due in part to the realization that the supply of hydrocarbon fuels, while fairly large, is not inexhaustible. Another factor that has contributed to the rapid development of this technology is the increasing concern about air and water pollution resulting from the burning of hydrocarbon fuels, as well as an increasing concern about a possible hazard connected with the use of nuclear fuels.
The recent development and growth of terrestrial solar cell technology has presented a need for producing low cost cells with fairly high efficiencies at a rather large rate. To meet this need, the solar cell industry has generally concentrated on producing sheets or films of low cost semiconductor materials from which the finished cells are fabricated, producing inexpensive processing methods for forming new types of solar cell junctions, and developing new types of cost-effective solar cell metallization processes. With regard to developing new metallization processes, thick film, metallization technology is currently recognized as a potentially cost-effective process for applying electrical metal contacts to the surfaces of various types of silicon solar cells.
Thick film technology may be broadly defined as that field of microelectronics in which specially formulated thick film inks are applied onto a ceramic or semiconductor substrate. The inks are applied in a definite pattern and sequence to produce a set of individual electrical components, such as, for example, resistors, capacitors, a complete functional circuit, or as in the case of solar cell metallization, back and front metal contacts. Conventional inks typically are high viscosity thixotropic pastes containing dielectric oxide, metal or ferroelectric oxide powders and a low softening glass frit or melted glass composition which is intermixed with an organic vehicle.
The inks are normally applied using a screen or mask method. When using the screen method with solar cells, the ink is usually forced through pre-processed openings in a stencil screen to deposit the required thick film contact pattern or system onto the semiconductor substrate. After depositing the contact pattern, the resultant thick film metal contact system is fired at elevated temperatures in either a periodic kiln or a belt furnace so as to mature the deposited contact system and to chemically bond the system integrally to the substrate.
The chemical bonding of the ink pattern to the substrate is facilitated by the frit contained in the ink and a sintering operation which occurs during firing. During sintering, the screened contact system is fired at a temperature sufficient to produce a coherent mass but less than that required to produce complete fusion. Additionally, during sintering the frit softens and flows to form a glaze-like glass film at an interface between the fired ink contact system and the substrate so as to bond the metal oxide powders of the ink to the substrate.
In addition to thick film technology providing a potentially cost-effective metallization process, it offers the advantage of providing a means for depositing the metal contacts onto the newly produced semiconductor cell only at positions where such contacts will be used. It also offers the advantage of providing a metal contact system of sufficient thickness to allow fairly high conductivity, as well as for providing a system that is substantially resistant to degradation by the terrestrial environment, is strongly adherent, that is easily automated.
Unfortunately, with respect to thick film solar cell metallization applications, the glaze-like glass film formed at the interface between the fired contact system and the cell substrate serves as a barrier for electron flow therebetween. This barrier creates an undesirable high contact resistance.
Contact resistance may be simply defined as the electrical resistance between the contact system and the cell surface. Since the contact resistance is one of the various sources or components of series cell resistance, an undesirable high contact resistance usually creates a corresponding undesirable high series cell resistance. The effect of the high series resistance is an undesirable flattening of the solar cell output characteristics and a related drop in its maximum power point voltage. Consequently, the high contact resistance created by the glass film barrier constitutes a severe disadvantage of the aforesaid metallization process.
Various approaches have been utilized by skilled technologists in the thick film, metallization, solar cell industry to reduce such contact resistance formed by the glass barrier, and thus to reduce the series resistance associated therewith. One approach commonly employed to reduce the series resistance is to etch the metal contact system with about 5% by weight of a hydrofluoric acid-type etchant for up to about 10 seconds almost immediately after firing. This approach makes use of the well known fact that the power output characteristics of a freshly fired solar cell are usually always undesirably low. However, upon etching with the hydrofluoric acid, the power output without exception is nearly always improved to satisfactory levels.
A serious drawback, however, with the acid etchant approach is that it introduces an extra process step to the conventional thick film metallization process, as well as additional equipment and labor normally required to accomplish the etching. Moreover, hydrofluoric acid-type etchants are extremely corrosive and hazardous, and thus must be handled carefully. For example, etching with hydrofluoric acid must be strictly controlled with respect to time.
Such control is required because the acid may substantially destroy the glaze-like glass film at the interface between the contact system and the cell substrate if the etching is performed for longer than a critical time. Destroying the glass film also undesirably destroys the adhesion created thereby. The requirement for controlling the time of the etch is complicated by the fact that for each ink an optimum etch time must be determined. The optimum etch time must be long enough to improve the cell output without destroying the adhesion formed by the frit. Additionally, a thorough washing of the cell is required after every hydrofluoric etching cycle to remove unwanted residue. Hence, for the aforesaid reasons the extra hydrofluoric acid etching operation is costly and undesirably time consuming.
Another prior art approach is to formulate the thick film ink composition having a minimum amount of frit contained therein. A significant shortcoming of formulating such a composition is that the resulting metal contact system is often marginally adherent.
Yet another prior art approach is to fire the screened thick film contact system using an infrared furnace. Using an infrared furnace for the firing cycle substantially decreases the firing time and causes less glass flow to the interface. Decreasing the firing time and causing less glass flow normally causes the contact resistance to be significantly reduced. However, utilizing an infrared furnace to fire the screened contact system also usually results in forming a contact system that is marginally adherent when exposed to the atmosphere. Additionally, the infrared approach requires a fairly costly expenditure for the infrared furnace.
The failure to solve satisfactorily the long-standing problem of the series resistance with the aforesaid prior art approaches has created an additional need either to offset or to reduce the cost of hydrofluoric acid etching or to find new or improved series resistance reduction techniques. Skilled theoreticians and researchers in the solar cell industry have addressed this latter need by reinvestigating solar cell manufacturing process operations.
The major manufacturing operations under investigation generally include the production and doping of semiconductor wafers, the fabrication of thick film metal contact systems, the application of hydrofluoric acid-type etchants, the fabrication of reflective coatings, and lastly, the packaging of metallized cells to form photovoltaic modules. Of the above operations, familiarity with hydrofluoric acid etching and packaging operations is required in order to appreciate fully the improved process of the present invention. The reasons for this requirement are twofold. First, the process of the present invention was developed out of theoretical and applied research on these by the applicants operations. Second, the process contemplates eliminating the hydrofluoric acid etching operation and altering the packaging operation in order to cope with the aforesaid need to reduce the undesirable cost associated with acid etching, as will be more fully explained hereinafter.
In conventional acid etching practices, as previously mentioned, the contact system is frequently etched with a hydrofluoric acid-type etchant and tested for its electrical characteristics by device engineers and technologists. In conventional packaging practices, packaging engineers and technologists normally electrically connect the metal contacts to other portions of the electrical system. Since the interconnection of the cells is primarily and routinely accomplished through soldering, it constitutes an important step thereof.
Conventional soldering, generally speaking, involves joining metals without using fasteners by employing a nonferrous metal whose melting point is below that of the base metal. The major soldering operations for thick film solar cell packaging typically involve initially cleaning the fired thick film solar cell by burnishing or applying a solvent cleaner. Cleaning is ordinarily followed by fluxing which involves removing oxide coatings and other contaminants from the metal surfaces to be assembled and lowering surface tensions of the solder, whereby the wetting or adhesion properties of the solder are increased. The next step is usually tinning. Tinning typically involves precoating the contact system with solder through either a solder dip or a solder wipe method. Lastly, the pretinned parts are usually heated and joined to one another without applying additional solder in a technique generally known as reflow assembling.
The present invention, as aforesaid, contemplates altering the soldering operation by employing a soldering flux, instead of hydrofluoric acid, as an etchant during the soldering cycle to reduce the series resistance. Heretofore, whenever an acid type etchant was used, the system was nearly always etched immediately following the earlier firing and metallization cycles of solar cell manufacture, as will be more fully explained hereinafter.
Prior art patents relating to the use of soldering fluxes in semiconductor manufacture, although not as an etchant to reduce the contact resistance formed by frit during sintering in the manufacture of thick film metallization solar cells, include U.S. Pat. Nos. 4,019,671; 3,574,925; 3,478,414; 3,295,196; 3,020,635; and 2,887,416. Additionally, some prior art patents relating to the soldering process used in the fabrication of printed circuit boards and other circuit systems include U.S. Pat. Nos. 4,196,839; 3,778,883; 3,680,762; 3,553,824; 3,484,929; 3,482,755; 3,053,699; and 3,035,339.